Magnetoencephalography (“MEG”) involves detecting magnetic fields produced within a subject's brain. Information about such biomagnetic fields is most useful when it can be associated with particular structures within the subject's brain to a spatial resolution of a few millimeters or better.
A typical MEG system comprises a helmet which carries a large number of magnetic detectors. The subject's head is placed inside the helmet. Magnetic signals originating from within the subject's head are detected at each of the magnetic detectors over a data acquisition period. The data acquisition period may, for example, be a few minutes.
Since biomagnetic signals are typically measured over significant time spans it is unreasonable to expect that a subject will be able to hold his or her head completely still throughout the measurement. Motions of the subject's head during the data acquisition period can interfere with the ability to associate particular magnetic signals with specific structures within the subject's brain. It is not practical to completely immobilize the subject's head.
One can localize the subject's head relative to the measured magnetic fields if the position of the subject's head relative to the magnetic detectors used to detect the magnetic fields is known. One way to localize a subject's head is to attach small coils at three or more known locations on the subject's head. The coils create fluctuating magnetic fields when alternating electrical currents are passed through them. Magnetic detectors are used to detect the coils' magnetic fields. The location of the coils, and thus the location of the subject's head, can be determined from the detected magnetic fields of the coils.
There is a need for biomagnetic measurement systems which can compensate for motion of the part of a subject being studied.